WO2025109158A1

WO2025109158A1 - Process for the chemical recycling of plastic waste containing polyethylene or polypropylene

Info

Publication number: WO2025109158A1
Application number: PCT/EP2024/083276
Authority: WO
Inventors: Manfred Julius
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2023-11-23
Filing date: 2024-11-22
Publication date: 2025-05-30
Anticipated expiration: 2026-05-23

Abstract

A process for the recycling of plastic waste containing at least one of polyethylene or polypropylene comprising the steps a) thermal pyrolysis in an inert atmosphere of the plastic waste to obtain a pyrolysis oil, b) optionally purifying the pyrolysis oil obtained in step a), c) fractionating the pyrolysis oil to obtain at least one fraction of lower boiling hydrocarbons that can be further processed in a cracker, in particular a steam cracker, to give hydrocarbons of lower molecular weight, and at least one fraction of high-boiling residues, d) incinerating high-boiling residues obtained in step c) with an oxygen containing gas, wherein a carbon dioxide containing flue gas stream is obtained, e) purifying the carbon dioxide containing flue gas stream obtained in step d), wherein a purified carbon dioxide containing gas stream is obtained, f) reduction of the carbon dioxide contained in the gas stream obtained in step e) to obtain a gas stream containing carbon monoxide, optionally carbon dioxide and optionally hydrogen, g) optionally admixing hydrogen, preferably produced by water electrolysis, to the gas stream obtained in step f), h) reacting a gas mixture containing carbon monoxide, hydrogen and optionally carbon dioxide obtained in step f) or g) to give methanol, i) manufacturing C₂-C₄-olefins by a methanol to olefin-process from methanol obtained in step h), j) polymerizing ethylene and/or propylene manufactured in step i) to give polyethylene and/or polypropylene, respectively.

Description

Process for the chemical recycling of plastic waste containing polyethylene or polypropylene

The present invention relates to processes for the chemical recycling of plastic waste containing at least one of polyethylene or polypropylene and optionally further polymers.

Municipal plastic waste (MPW) is usually comprised of 6 main types of plastics: high-density polyethylene (HDPE), low-density polyethylene (LDPE), polystyrene (PS), polypropylene (PP), polyvinylchloride (PVC), and polyethylene terephthalate (PET). PET, for example, is very often used in the production of plastic bottles, and PVC due to its combustibility resistance is mainly used for building purposes and in furniture. However, PS can be found in food and industrial packaging and insulation materials. On the other hand, polyolefins, starting with PP can be found in water or sewage pipes and other household equipment’s, and PE, the most produced plastic, is used for plastic bags (LDPE) and many other applications as glass replacement and irrigation pipes (HDPE). See Jaafar et aL, Polymer Degradation and Stability 195 (2022) 109770.

Based on the works of Roosen et aL, Environ. Sci. TechnoL 54 (20), 13282-13293 and Klein- hans et aL, Waste Manag. 120, 290-302, the MPW fraction is largely composed of polyolefins (i.e. , PE and PP) with considerable amounts of PET, polystyrene and other polymers such as ethylene vinyl alcohol (EVOH), polyamides (PA), PVC, polyurethanes (PUR) and ethylene vinyl acetate (EVA).

Mechanical recycling is most implemented at this point of time, but in fact, it is known that plastics cannot be infinitely mechanically recycled without deterioration of their properties. A promising alternative is chemical recycling with pyrolysis being considered the dominant technology in the coming decades. In pyrolysis, polymer waste is decomposed into primarily a liquid product by providing thermal energy. To close the material loop, the pyrolysis oils need to be subsequently cracked, in particular steam cracked, to obtain base chemicals for the production of second-generation virgin-quality polymeric materials. Other suitable cracking processes include thermal cracking, fluid catalytic cracking or hydrocracking.

It is an object of the present invention to provide an environmentally friendly process for the recycling of plastic waste, in particular municipal plastic waste, containing polyethylene and/or polypropylene. In particular, it is an object of the present invention to provide such a recycling process that closes the recycle loop by producing again (virgin) polyethylene and/or polypropylene. It is a further object of the present invention to provide such a recycling process that effectively uses most or essentially all of the carbon contained in the process streams, in order to produce polyethylene and/or polypropylene, thereby minimizing the carbon footprint of the overall process. The object is achieved by a process for the recycling of plastic waste containing at least one of polyethylene or polypropylene comprising the steps: a) thermal pyrolysis in an inert atmosphere of the plastic waste to obtain a pyrolysis oil, b) optionally purifying the pyrolysis oil obtained in step a), c) fractionating the pyrolysis oil to obtain at least one fraction of lower boiling hydrocarbons that can be further processed in a cracker, preferably a steam cracker, to give hydrocarbons of lower molecular weight, and at least one fraction of high-boiling residues, d) incinerating high-boiling residues obtained in step c) with an oxygen containing gas, wherein a carbon dioxide containing flue gas stream is obtained, e) purifying the carbon dioxide containing flue gas stream obtained in step d), wherein a purified carbon dioxide containing gas stream is obtained, f) reduction of the carbon dioxide contained in the gas stream obtained in step e) to obtain a gas stream containing carbon monoxide, optionally carbon dioxide and optionally hydrogen, g) optionally admixing hydrogen, preferably produced by water electrolysis, to the gas stream obtained in step f), h) reacting a gas mixture containing carbon monoxide, hydrogen and optionally carbon dioxide obtained in step f) or g) to give methanol, i) manufacturing ethylene and/or propylene by a methanol to olefin-process from methanol obtained in step h), j) polymerizing ethylene and/or propylene manufactured in step i) to give polyethylene and/or polypropylene, respectively.

The object is further achieved by an alternative process for the recycling of plastic waste containing at least one of polyethylene or polypropylene comprising the steps: a) thermal pyrolysis in an inert atmosphere of the plastic waste to obtain a pyrolysis oil, b) optionally purifying the pyrolysis oil obtained in step a), c) fractionating the pyrolysis oil to obtain at least one fraction of lower boiling hydrocarbons that can be further processed in a cracker, preferably a steam cracker, to give hydrocarbons of lower molecular weight, and at least one fraction of high-boiling residues, d) incinerating high-boiling residues obtained in step c) with an oxygen containing gas, wherein a carbon dioxide containing flue gas stream is obtained, e) purifying the carbon dioxide containing flue gas stream obtained in step d), wherein a purified carbon dioxide containing gas stream is obtained, f) admixing hydrogen, preferably produced by water electrolysis, to the gas stream obtained in step e), g) reacting a gas mixture containing hydrogen and carbon dioxide obtained in step f) to give ethylene and/or propylene, or h) reacting a gas mixture containing hydrogen and carbon dioxide obtained in step f) to give methanol, and i) manufacturing ethylene and/or propylene by a methanol to olefin-process from methanol obtained in step h), j) polymerizing ethylene and/or propylene manufactured in step g) or i) to give polyethylene and/or polypropylene, respectively.

In step a) of the process of the invention, the plastic waste containing at least one of polyethylene or polypropylene is subjected to thermal pyrolysis.

The plastic waste containing at least one of polyethylene or polypropylene can contain in addition one or more polymers selected from PET, PS, EVOH, PA, PVC, PUR and EVA. It is preferable to concentrate polyethylene- and polypropylene-based plastic waste by sorting out other types of plastic waste, in particular plastic waste based on chlorine containing polymers, such as PVC, and nitrogen containing polymers, such as polyurethanes. Preferably 5 to 85 % by weight, more preferably 10 to 80 % by weight, e.g. 25 to 75 % by weight, of these other types of plastic waste are sorted out.

Preferably, the plastic waste fed to the thermal pyrolysis (step a)) contains > 50, more preferably > 75, in particular > 80 % by weight, and < 98, more preferably < 95, in particular < 90 % by weight of polyethylene and/or polypropylene.

Pyrolysis is a thermochemical process in which the feedstock is decomposed to smaller molecules using heat under an inert atmosphere. One of the pioneering studies on the pyrolysis of plastic waste and its potential was made by Scott et aL, Fast pyrolysis of plastic wastes. Energy Fuels 1990, 4, 407-411 . The product obtained from pyrolysis relies on several factors ranging from the type of plastic, reactor used, operating conditions (temperature, pressure, residence time, and heating rate), and the use of a catalyst. See Miandad, R et aL, Effect of plastic waste types on pyrolysis liquid oil, Int. Biodeterior. Biodegrad. 2016, 119, 239-252; Butler, E et aL, Waste Polyolefins to Liquid Fuels via Pyrolysis: Review of Commercial State-of-the-Art and Recent Laboratory Research, Waste Biomass Valorization 2011 , 2, 227-255; and Miandad, R. et aL, Catalytic pyrolysis of plastic waste: A review. Process. Saf. Environ. Prot. 2016, 102, 822-838. The influence of these operational parameters is assessed in several articles, including: Arena, LL; Mastellone, M. Fluidized bed pyrolysis of plastic wastes. In Feedstock Recycling and Pyrolysis of Waste Plastics: Converting Waste Plastics into Diesel and Other Fuels; Wiley: Chichester, West Sussex, UK, 2006; pp. 435-474; Butler, E. et aL: A review of recent laboratory research and commercial developments in fast pyrolysis and upgrading. Renew. Sustain. Energy Rev. 2011 , 15, 4171-4186; Encinar, J. et aL, Pyrolysis of synthetic polymers and plastic wastes. Kinetic study. Fuel Process. TechnoL 2008, 89, 678-686; Lopez, A. et aL, Influence of time and temperature on pyrolysis of plastic wastes in a semi-batch reactor. Chem. Eng. J. 2011 , 173, 62-71 ; and Qureshi, M.S. et aL, J. Pyrolysis of plastic waste: Opportunities and challenges. J. Anal. AppL Pyrolysis 2020, 152, 104804.

The hydrocarbon range obtained is significantly altered by the variation of these influencing parameters. Fast pyrolysis of plastic waste is normally carried out in fluidized bed reactor and the residence time is nearly 1 s. The primary product obtained by fast pyrolysis of plastics at moderate temperatures (around 500 °C) is wax, which contains molecules with a carbon number above C20. The influence of residence time and temperature is significant in the modification of the pyrolysis yields and product selectivity, see Singh, R.K. et aL, Impact of fast and slow pyrolysis on the degradation of mixed plastic waste: Product yield analysis and their characterization. J. Energy Inst. 2019, 92, 1647-1657. Mastral, F.J. et aL, Pyrolysis of high-density polyethylene in a fluidised bed reactor. Influence of the temperature and residence time. J. Anal. AppL Pyrolysis 2002, 63, 1-15; and Onwudili, J. et aL, Composition of products from the pyrolysis of polyethylene and polystyrene in a closed batch reactor: Effects of temperature and residence time. J. Anal. AppL Pyrolysis 2009, 86, 293-303.

Depending on the feedstock and the pyrolysis process conditions, the yields of gas, condensables (pyrolysis oil and wax), and solid (char) products differ. Moderate temperatures (around 500 °C), fast heating rates and short vapor residence times generally result in a greater yield of pyrolysis oil/wax than gases and char. On the contrary, low temperatures, slow heating rates and long vapor residence time generally result in an improved yield of char at the expense of pyrolysis oil yield. The liquid product from pyrolysis of plastics typically contains heavy oil, light oil, mid-distillates, and naphtha. Depending on the feedstock, the heavier oils contain paraffins, olefins, aromatics, and high molecular weight components, typically with boiling points greater than 250 °C. This wax-like fraction may appear solid at room temperature. Pyrolysis of polyethylene (PE), for instance, generated a relatively large proportion of wax at low pyrolysis temperature (450 °C). On the other hand, a higher temperature (above 600 °C) resulted in wax breaking down into shorter components, together with an enhanced gas formation. The liquid product from waste plastics pyrolysis can potentially be utilized as a feed stock in the petrochemical industry, where it for example replaces fossil naphtha to produce olefins via the steam cracking process. In general, the thermal pyrolysis in step a) is carried out at a temperature in the range from 300 to 550 °C.

In step b) of the process of the invention, the pyrolysis oil obtained in step a) is purified. In general, the pyrolysis oil obtained in step a) is purified in order to at least partly remove inorganic compounds and/or heteroatom containing organic compounds. Heteroatom containing organic compounds are in general those containing one or more of O, N, S, P, Si or halogen, in particular Cl.

Purification of pyrolysis oil is described inter alia in WO 2022/219045 A1 , WO 2023/073059 A1 , WO 2023/072644 A1 , WO 2023/061834 A1 , WO 2023/061798 A1 , and WO 2021/224287 A1 . In general, purification of pyrolysis oil comprises a water washing step to remove water soluble compounds and/or a hydrotreatment step (hydrotreatment: reaction with hydrogen in the presence of a catalyst). Either or both steps can be present in a suitable purification process. An overview of hydrotreatment is given in Emmanuel Ortega, aiche.org/cep, October 2021 , pages 29 to 33.

In step c) of the process of the invention, the pyrolysis oil obtained in step a) or b) is fractionated to obtain at least one fraction of lower boiling hydrocarbons that can be further processed in a cracker, in particular a steam cracker, to give hydrocarbons of lower molecular weight, and at least one fraction of high-boiling residues. A suitable process is described in WO 2011/133875 A1.

The fraction of high-boiling residues obtained in step c) to be incinerated in step d) can be a wax-like fraction of high molecular weight components, typically with boiling points higher than 250 °C at atmospheric pressure. The fraction of high-boiling residues obtained in step c) to be incinerated in step d) can be char and usually contains char.

Fraction of lower boiling hydrocarbons that are fed to a steam cracker are preferably naphtha with a boiling range of from 30 to 250 °C, preferably 30 to 180 °C, according to DIN EN ISO 3405 or ASTM D 86. Higher boiling residues can be incinerated according to the invention.

In one preferred embodiment of the invention, lower boiling hydrocarbons obtained in step c) are further processed in a cracker, preferably a steam cracker, to give ethylene and/or propylene. The steam cracking process is described in detail by Heinz Zimmermann and Roland Walzl in Ullmann’s Encyclopedia of Industrial Chemistry, Vol. 13, Ethylene, pages 465 - 529, with further references.

According to one embodiment of the invention, ethylene and/or propylene obtained thereby are polymerized to give polyethylene and/or polypropylene, respectively. In this way, the recycle loop is closed. In step d) of the process of the invention, the high-boiling residues obtained in step c) are incinerated, wherein a carbon dioxide containing flue gas stream is obtained.

Preferably in step d), the high-boiling residues obtained in step c) are incinerated with essentially pure oxygen O2, for example, which is evolved at the anode as by-product of the (preferred) CO2 electrolysis in step f) or the optional water electrolysis. The heat of reaction resulting from the incineration in step d) can be used to produce steam and electrical current. In particular, the heat can be used to operate the pyrolysis in step a) and the electrical current generated can be used in reduction step f) if it is carried out as electrochemical reduction of carbon dioxide (carbon dioxide electrolysis). The electrical current generated can also be used in the electrolysis of water to produce hydrogen and oxygen. This further improves the efficiency of the novel overall method.

Thus, in one preferred embodiment of the inventive process, the high-boiling residue are incinerated in step d) with essentially pure oxygen as oxygen containing gas. Essentially pure in the context of the present invention means an oxygen content of at least 80 vol.-%, preferably at least 90 vol.-%.

The CO2 originating from the incineration in step d) can thus be obtained in highly concentrated form and is fed to purification step e) before further use. In this purification process, the by-products of the incineration, for example sulfur compounds such as SO2, nitrogen compounds such as NO_X and residual organics a well as dust and other compounds formed from the components present in the plastic waste material, are separated.

The incineration of the high-boiling residues with essentially pure oxygen according to step d) can be carried out, for example, according to the process known as the oxy-fuel process in an atmosphere of pure oxygen and CO2 (recirculating flue gas). The resulting flue gas is not diluted with the nitrogen present in air and consists essentially of CO2 and water vapor. The water vapor can be easily condensed, so that a highly concentrated CO2 stream is formed. The CO2 can then be purified and further processed, optionally also compressed and stored.

Furthermore, some of the energy obtained from the incineration in step d) of the plastic waste material can be converted into steam or electricity. The electricity obtained can be used to operate the (preferred) carbon dioxide electrolysis in step f), resulting in an even more efficient process with low consumption of electrical energy. The electricity obtained can also be used in water electrolysis to generate hydrogen and oxygen.

In a preferred embodiment of the inventive process, heat generated in incineration step d) is used to produce electrical power.

In step e) of the process of the invention, the carbon dioxide containing flue gas stream obtained in step d) is purified, wherein a purified carbon dioxide containing gas stream is obtained. The purification of CO2 from combustion gases can be carried out using methods known in principle from the prior art. First, for example, the combustion gases are purified, the main component of which is CO2. The assembly of a combustion gas purification system is divided into different stages. The particular task of the purification is to provide CO2 without secondary constituents that disrupt the subsequent preferred electrochemical reduction of CO2 at a gas diffusion electrode, as described below.

In the first stage, dust is removed from the combustion gas. This can be done with fabric filters or with an electrostatic filter. Any acidic gas present, such as hydrogen chloride, which is formed from chlorine compounds present in the waste, can then be removed. Offgas scrubbing towers are used here, for example. The combustion gas is also cooled here and freed from further dusts and possibly heavy metals. In addition, sulfur dioxide gas formed is also separated off in a scrubbing circuit and converted into gypsum, for example with hydrated lime. The removal of nitrogen compounds from the combustion gases can be carried out, for example, on catalyst-containing zeolites or by adding urea or ammonia, to convert the nitrogen oxides back to nitrogen and water. In order to prevent the formation of ammonium salts, which would clog the pores of the catalyst, the catalysts are usually operated at a temperature of above 320 °C. Likewise, the nitrogen compounds can be removed by scrubbing with nitric acid or with catalysts.

The CO2 can be dried and further purified by known conventional methods. Drying, for example, is possible by treatment with concentrated sulfuric acid.

In the final purification stage, activated carbon filters can be used to remove residual organics and any last metal residues from the combustion gas using activated carbon. For this purpose, for example, activated carbon in the form of dust can be metered into the combustion gas stream or flue gas stream and then deposited again on the fabric filter together with the accumulated pollutants. The spent carbon is discharged and fed to energy recovery.

CO2 can further be concentrated (removed from inert gases) by means of amine scrubbing from gas streams with a lower concentration of CO2. Absorption, or carbon dioxide scrubbing, with amines is the dominant capture technology (e. g. BASFs OASE® process).

CO2 can also be adsorbed to a MOF (Metal-organic framework) through physisorption or chemisorption based on the porosity and selectivity of the MOF leaving behind a CO2 poor gas stream. The CO2 is then stripped off the MOF using temperature swing adsorption (TSA) or pressure swing adsorption (PSA) so the MOF can be reused.

After the purification processes of the combustion gases have been carried out, CO2 which can be used as feedstock in step f) is available.

In step f) of the process of the invention, reduction of the carbon dioxide contained in the gas stream obtained in step e) is carried out to obtain a gas stream containing carbon monoxide, optionally carbon dioxide and optionally hydrogen. Reduction of the carbon dioxide contained in the gas stream obtained in step e) is preferably carried out electrochemically. However, reduction of carbon dioxide can be also carried out with hydrogen (H₂) or with carbon (CO₂ + C -> 2 CO) e.g. in a plasma process. See https://pubs.acs.org/doi/10.1021/acscatal.1c05358; and ACS CataL 2022, 12, 4, 2561-2568 Publication Date: February 4, 2022, https://doi.org/10.1021/acscatal.1c05358.

In case that reduction of carbon dioxide is carried out as electrochemical reduction to obtain a gas stream containing carbon monoxide, optionally carbon dioxide and optionally hydrogen, a gas stream containing oxygen is also obtained.

A large number of methods for the electrochemical production of CO from CO₂ have been proposed. Most of these methods are at a very early stage of development. High-temperature electrolysis in solid oxide cells is a CO₂ electrolysis technology that is approaching commercialization and for which long-term durability exceeding a year on stream has been demonstrated. A non-exhaustive review of two alternative electrochemical technologies for CO production, low- temperature and molten carbonate electrolysis, is given in Reiner Kungas 2020 J. Electrochem. Soc. 167 044508.

Regardless of the choice of technology, an electrolysis cell always has at least three components: two electrodes in contact with an electrolyte. The electrolyte is either a liquid or a solid material that can conduct ions (e.g. protons, hydroxide ions, oxide ions, carbonate or bicarbonate ions), but that is impermeable to electrons. The ionic conductivity of the electrolyte depends strongly on temperature and the choice of the electrolyte material thereby determines the operating temperature of the cell. When an external voltage is applied between the two electrodes, electrochemical reactions start to occur. The electrode where the reduction of reactants (e.g. CO₂ to CO) takes place is called the cathode. The electrode where the oxidation of reactants (e.g. OH- to O₂ and H₂O or O²- to O₂) occurs is referred to as the anode.

In solid oxide electrolysis cells (SOECs), the electrolyte is a solid ceramic material. At temperatures above around 600 °C, electrolyte materials start to conduct oxide ions, but remain impermeable to gaseous oxygen and to electrons. As the ionic conductivity of electrolyte materials increases exponentially with temperature, the operating temperature of SOECs is typically chosen to be between 700 °C and 900 °C. Commonly used materials include stabilized zirconias, such as yttria-stabilized zirconia (YSZ, a solid solution of Y₂O₃ and ZrO₂) and scandia-stabilized zirconia (ScSZ), as well as doped cerias, such as gadolinia-doped ceria (abbreviated either as GDC or CGO) or samaria-doped ceria (SDC or CSO).

CO₂ is fed to the cathode side of the cell via gas channels, which help to distribute the gas across the cell. In the porous cathode (also referred to as the fuel electrode), carbon dioxide is reduced to carbon monoxide, following the reaction

C0₂ + 2e- -> CO + O²- The electrons for the reaction are provided by an external power supply. The oxide ions (O²-) formed in the reaction are incorporated into the electrolyte and traverse through the electrode into the anode (also called the oxygen electrode), where the ions are oxidized into molecular oxygen according to the reaction

O²- -> ¹/₂ O₂ + 2 e-

The formed oxygen gas is led out of the cell via gas channels. It is important to note that as long as pure CO₂ (or a mixture of CO and CO₂) is fed to the fuel electrode, the formed product will be free of H₂ and H₂O.

Composites of metallic Ni and either CGO or YSZ are the most commonly used materials in SOEC fuel electrodes. In these electrodes, Ni acts as an electronic conductor and catalyst, while the electrolyte material provides ionic conductivity and helps to stabilize the electrode microstructure. Typical oxygen electrode materials for SOECs include doped perovskites of lanthanides and transition metals, such as Sr-doped LaMnOs (LSM), Sr-doped La(Fe,Co)O3 (LSCF), Sr-doped SmCoOs (SSC) and many others.

In molten carbonate electrolysis cells (MCECs), the electrolyte is a carbonate melt. A combination of molten Li₂O/Li₂CO3 electrolyte, a titanium cathode and a graphite anode has been shown to give promising results. In this material system, carbonate ions are reduced to CO and oxide ions at the cathode according to reaction

CO₃ ²- + 2 e- -> CO + 2 O²- while oxide ions are oxidized to gaseous oxygen at the anode:

O²- -> ¹/₂ O₂ + 2 e-

In other words, Li₂COs is electrochemically converted into Li₂O on the cathode, thereby increasing the Li₂O/Li₂CO3 ratio in the melt. As the oxide content in the electrolyte increases, new CO₂ can chemically incorporated into the mixture. The ratio of Li₂O/Li₂CO3 in the electrolyte is thereby a function of both by the applied current density and the concentration of CO₂ above the melt. A key advantage of MCECs is that the CO₂ feed and the CO and O₂ products do not mix, allowing pure gases to be extracted from the cell. Additionally, the method is only mildly affected by SO₂-content in the feed gas, and can potentially use dilute and humid CO₂ streams, suggesting that industrial flue gases may be used as feed.

In low-temperature electrolysis cells, CO₂ reduction is carried out in aqueous solutions. The electrolytes can either be solid ion-selective membranes (e.g. Nation, Sustainion), aqueous solutions (e.g. KHCO3), or combinations thereof. Most of the low temperature electrolysis cells to- day operate in alkaline or pH-neutral conditions. It should be noted at industrially relevant current densities (>200 mA cm-²), the local pH near the cathode will inevitably be very alkaline, even if neutral electrolytes are used.

Most of the electrode development work on electrochemical CO2 reduction has been carried out using a cell configuration where both electrolyzer electrodes are immersed in electrolyte solutions (the anolyte and catholyte, respectively). Such an electrode configuration is referred to as the H-celL On the anode of such a cell, oxygen evolution proceeds according to either of the two reactions:

2 OH- -> ¹/₂ O₂ + H₂O + 2 e- H₂O -> ¹/₂ O₂ + 2 H⁺ + 2 e-

On the cathode, CO2 is electrochemically reduced to CO:

CO₂ + H₂O + 2 e- -> CO + 2 OH-

Commonly, CO production is accompanied by hydrogen evolution, which in alkaline media proceeds via reaction

2 H₂O + 2 e- -> 2 OH- + H₂

The delivery of gas-phase CO2 to the cathode and the use of gas-diffusion electrodes present means of overcoming mass transport limitations in low-temperature electrolysis systems. In some designs, gas-diffusion electrodes are employed in both electrodes. lrO2 is used almost exclusively as the catalyst material on the anode side of aqueous electrolysis cells. Cathode materials for the production of CO typically include Ag and Au, with catalyst supports shown to play an important role for activity, selectivity, and stability.

The electrical power necessary for the electrochemical reduction of CO2 to CO is generated at least in part from non-fossil, renewable resources. In other words, part of the electrical power can still be produced from fossil fuels, preferably from natural gas, since combustion of natural gas causes much lower carbon dioxide emission per Megajoule of electrical energy produced than combustion of coal. However, the portion of electrical energy produced from fossil fuels should be as low as possible, preferably < 50%, preferably < 30%, most preferably < 20%.

The electrical power from non-fossil resources used in carbon dioxide or water electrolysis according to the invention can be generated by nuclear energy. Nuclear energy is considered renewable by the European Commission, as long as certain preconditions (i. a. safe long-term storage of nuclear waste) are fulfilled. The electrical power from non-fossil resources used in carbon dioxide or water electrolysis according to the invention is preferably generated from wind power, solar energy, biomass, hydropower and geothermal energy.

In one preferred embodiment of the inventive process, the electrical power used in carbon dioxide or water electrolysis is generated from hydropower. There are many forms of hydropower. Traditionally, hydroelectric power comes from constructing large hydroelectric dams and reservoirs. Small hydro systems are hydroelectric power installations that typically produce up to 50 MW of power. They are often used on small rivers or as a low-impact development on larger rivers. Run-of-the-river hydroelectricity plants derive energy from rivers without the creation of a large reservoir. The water is typically conveyed along the side of the river valley (using channels, pipes and/or tunnels) until it is high above the valley floor, whereupon it can be allowed to fall through a penstock to drive a turbine.

Wave power, which captures the energy of ocean surface waves, and tidal power, converting the energy of tides, are two forms of hydropower with future potential.

In one further preferred embodiment of the inventive process, the electrical power used in electrolysis is generated at least in part from geothermal energy. Geothermal energy is the heat that comes from the sub-surface of the earth. It is contained in the rocks and fluids beneath the earth’s crust and can be found as far down to the earth’s hot molten rock, magma. To produce power from geothermal energy, wells are dug a mile deep into underground reservoirs to access the steam and hot water there, which can then be used to drive turbines connected to electricity generators. There are three types of geothermal power plants; dry steam, flash and binary. Dry steam is the oldest form of geothermal technology and takes steam out of the ground and uses it to directly drive a turbine. Flash plants use high-pressure hot water into cool, low-pressure water whilst binary plants pass hot water through a secondary liquid with a lower boiling point, which turns to vapor to drive the turbine.

In one further preferred embodiment of the inventive process, the electrical power used in carbon dioxide or water electrolysis is generated from wind power. Wind power can be used to run wind turbines. Modern utility-scale wind turbines range from around 600 kW to 9 MW of rated power. The power available from the wind is a function of the cube of the wind speed, so as wind speed increases, power output increases up to the maximum output for the particular turbine. Areas where winds are stronger and more constant, such as offshore and high-altitude sites, are preferred locations for wind farms.

In one further preferred embodiment of the inventive process, the electrical power used in carbon dioxide or water electrolysis is generated from solar power, particularly preferred from photovoltaic systems. A photovoltaic system converts light into electrical direct current (DC) by taking advantage of the photoelectric effect. Concentrated solar power (CSP) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. CSP-Stirling has by far the highest efficiency among all solar energy technologies. In one further preferred embodiment of the inventive process, the electrical power used in carbon dioxide or water electrolysis is generated from biomass. Biomass is biological material derived from living, or recently living organisms. It most often refers to plants or plant-derived materials which are specifically called lignocellulosic biomass. As an energy source, biomass can either be used directly via combustion to produce heat or electricity, or indirectly after converting it to various forms of biofuel. Conversion of biomass to biofuel can be achieved by different methods which are broadly classified into: thermal, chemical, and biochemical methods. Wood was the largest biomass energy source as of 2012; examples include forest residues - such as dead trees, branches and tree stumps -, yard clippings, wood chips and even municipal solid waste. Industrial biomass can be grown from numerous types of plants, including miscanthus, switchgrass, hemp, corn, poplar, willow, sorghum, sugarcane, bamboo, and a variety of tree species, ranging from eucalyptus to oil palm (palm oil).

In optional step g) of the process of the invention, hydrogen that is preferably produced by water electrolysis is admixed to the gas stream obtained in step f).

As described above, the carbon monoxide containing gas stream can be either pure carbon monoxide or can contain hydrogen and possibly also carbon dioxide. Low-temperature electrolysis cells, wherein CO2 reduction is carried out in alkaline aqueous solution, yields a carbon monoxide containing gas stream containing also hydrogen.

Preferably, electrolysis of water is carried out using electrical power generated at least in part from non-fossil energy, as described above.

Electrolysis of water is an environmentally friendly method for production of hydrogen because it uses renewable H2O and produces only pure oxygen as by-product. Additionally, water electrolysis utilizes direct current (DC) from sustainable energy resources, for example solar, wind, hydropower and biomass.

It is observed that by electrolysis of water, the deuterium atom content of the hydrogen is lower than in the hydrogen generated petrochemically, for example as contained in synthesis gas, in general < 100 ppm, preferably in general < 90 ppm, for example from 30 to 75 ppm.

One suitable water electrolysis process is alkaline water electrolysis. Hydrogen production by alkaline water electrolysis is a well established technology up to the megawatt range for a commercial level. In alkaline water electrolysis initially at the cathode side two water molecules of alkaline solution (KOH/NaOH) are reduced to one molecule of hydrogen (H2) and two hydroxyl ions (OH-). The produced H2 emanates from the cathode surface in gaseous form and the hydroxyl ions (OH-) migrate under the influence of the electrical field between anode and cathode through the porous diaphragm to the anode, where they are discharged to half a molecule of oxygen (O2) and one molecule of water (H2O). Alkaline electrolysis operates at lower temperatures such as 30-80°C with alkaline aqueous solution (KOH/NaOH) as the electrolyte, the concentration of the electrolyte being about 20% to 30 %. The diaphragm in the middle of the electrolysis cell separates the cathode and anode and also separates the produced gases from their respective electrodes, avoiding the mixing of the produced gases. However, alkaline electrolysis has negative aspects such as limited current densities (below 400 mA/cm²), low operating pressure and low energy efficiency.

In one preferred embodiment of the inventive process, hydrogen is provided by polymer electrolyte membrane water electrolysis. Variants of polymer electrolyte membrane water electrolysis are proton exchange membrane water electrolysis (PEMWE) and anion exchange membrane water electrolysis (AEMWE).

PEM water electrolysis was developed to overcome the drawbacks of alkaline water electrolysis. PEM water electrolysis technology is similar to the PEM fuel cell technology, where solid polysulfonated membranes (Nation®, fumapem®) are used as an electrolyte (proton conductor). These proton exchange membranes have many advantages such as low gas permeability, high proton conductivity (0.1 ± 0.02 S cm^-1), low thickness (20-300 pm), and allow high-pressure operation. In terms of sustainability and environmental impact, PEM water electrolysis is one of the most favorable methods for conversion of renewable energy to highly pure hydrogen. PEM water electrolysis has great advantages such as compact design, high current density (above 2 A cm^-2), high efficiency, fast response, operation at low temperatures (20-80°C) and production of ultrapure hydrogen. The state-of-the-art electrocatalysts for PEM water electrolysis are highly active noble metals such as Pt/Pd for the hydrogen evolution reaction (HER) at the cathode and lrO2/ uC>2 for the oxygen evolution reaction (OER) at the anode.

One of the largest advantages of PEM water electrolysis is its ability to operate at high current densities. The PEM water electrolyzer utilizes a solid polymer electrolyte (SPE) to conduct protons from the anode to the cathode while insulating the electrodes electrically. Under standard conditions the enthalpy required for the formation of water is 285.9 kJ/mol. One portion of the required energy for a sustained electrolysis reaction is supplied by thermal energy and the remainder is supplied through electrical energy.

The half reaction taking place on the anode side of a PEM water electrolyzer is commonly referred to as the Oxygen Evolution Reaction (OER). Here the liquid water reactant is supplied to a catalyst where it is oxidized to oxygen, protons and electrons.

The half reaction taking place on the cathode side of a PEM water electrolyzer is commonly referred to as the Hydrogen Evolution Reaction (HER). Here the protons that have moved through the membrane are reduced to gaseous hydrogen.

PEMs can be made from either pure polymer membranes or from composite membranes, where other materials are embedded in a polymer matrix. One of the most common and commercially available PEM materials is the fluoropolymer PFSA, or Nation®, a DuPont product. While Nation® is an ionomer with a perfluorinated backbone like Teflon, there are many other structural motifs used to make ionomers for proton-exchange membranes. Many use polyaromatic polymers, while others use partially fluorinated polymers.

An overview over hydrogen production by PEM water electrolysis is given in S. Kumar and V. Himabindu, Material Science for Energy Technologies 2 (2019), pp. 4442 - 4454.

An overview over hydrogen production by anion exchange membrane water electrolysis is given in H. A. Miller et aL, Sustainable Energy Fuels, 2020, 4, pp. 2114 - 2133.

In step h) of the process of the invention, a gas mixture containing carbon monoxide, hydrogen and carbon dioxide obtained in step f) or g) is reacted to give methanol.

The current world-scale technology for methanol synthesis is mostly based on the application of Cu/ZnO/AI2O3 (CZA) catalysts in either multi-tube reactors with boiling water as the cooling fluid, normally called isothermal reactors (e.g., the Lurgi process, the Linde process), or adiabatic reactors with intermediate cold syngas quenching, generally named quench reactors (e.g., ICI and the Casale process, the Haldor Topsoe process). Less common but also industrially applied are the adiabatic reactors with intermediate cooling (e.g., the Kellogg process, the Toyo process). Normally, temperatures between 200 and 300 °C and pressures between 50 and 100 bar (abs) are applied. See Bozzano, G.; Manenti, F. Efficient methanol synthesis: Perspectives, technologies and optimization strategies. Prog. Energy Combust. Sci. 2016, 56, 71-105; and Ott, J.; Gronemann, V.; Pontzen, F.; Fiedler, E.; Grossmann, G.; Kersebohm, D.B.; Weiss, G.; Witte, C. Methanol. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley: New York, NY, USA, 2012.

In step i) of the process of the invention, ethylene and/or propylene are manufactured by a methanol to olefin-process (MTO-process) from methanol obtained in step h) See ACS CataL 2015, 5, 1922-1938, DOI: 10.1021 /acscatal.5b00007. Preferably, ethylene or propylene are manufactured in step i). Of course, both ethylene and propylene can be manufactured in step i).

In a further step j), ethylene and/or propylene are polymerized to give polyethylene and/or polypropylene, respectively, to close the recycle loop.

In an alternative process of the invention, in step f) hydrogen, preferably produced by water electrolysis, is admixed to the gas stream obtained in step e), and in step g) the gas mixture containing hydrogen and carbon dioxide obtained in step f) is directly reacted to give ethylene and/or propylene, or in step h) the gas mixture containing hydrogen and carbon dioxide obtained in step f) is reacted to give methanol, and in step i) ethylene and/or propylene are manufactures by a methanol to olefin-process from methanol obtained in step h). Ethylene or propylene are manufactured in step g) or step I). Of course, both ethylene and propylene can be manufactured in steps g) or i).

In step g) of this alternative process, hydrogen and carbon dioxide obtained in step f) are directly reacted to give C2-C4-olefins. This process is described by Jianli Zhang et aL, Selective formation of light olefins from CO2 hydrogenation over Fe-Zn-K catalysts, Journal of CO2 Utilization Volume 12, December 2015, pages 95-100.

In step h) of this alternative process, which can be carried out in addition to step g) or instead of step g), hydrogen and carbon dioxide are reacted in the presence of a catalyst to form methanol.

An overview of suitable catalyst systems is given by Kristian Stangeland, Hailong Li & Zhixin Yu, Energy, Ecology and Environment volume 5, pages 272-285 (2020). Multi-component catalyst systems are required for this process. The interaction between components is essential for high activity and selectivity of CC>2-to-methanol catalysts. This has been demonstrated by numerous catalyst systems comprised of various metals (i.e., Cu, Pd, Ni) and metal oxides (i.e., ZnO, ZrC>2, ln₂O₃). These complex systems can contain a mixture of metallic, alloy, and metal oxide phases. The most promising catalyst systems for large-scale industrial processes are currently Cu-based and In-based catalysts due to their superior catalytic performance.

A process for the CC>2-to-methanol synthesis can be carried out, for example, by the method known from DE-A-42 20 865, which produces methanol under the influence of silent electrical discharges.

Alternatively, methanol synthesis can also be carried out in a thermal reactor under pressure and elevated temperature and in the presence of a copper-based catalyst, as described in DE 43 32 789 A1 and DE 19739773 A1 .

Typical catalysts are described, for example, in the publication by N.Kanoun et al. "Catalytic properties of Cu based catalysts containing Zr and/or V for methanol synthesis from a carbon dioxide and hydrogen mixture" in CATALYSIS LETTERS 15,(1992) 231-235. Potential catalysts like CuO/ZnO and Cu-ZnO-AhOs are also described by R. M. Navarro et al. “Methanol Synthesis from CO2: A Review of the Latest Developments in Heterogeneous Catalysis” Materials (2019), 12, 3902 and in “Catalytic carbon dioxide hydrogenation to methanol: A review of recent studies” in chemical engineering research and design 92 (2014) 2557-2567.

Recently a high selective catalysts ln2O3/ZrO2 was described for industrial relevant conditions. A typical range of industrially relevant conditions for the hydrogenation of CO2 to methanol are T = 200 - 300°C, p = 10 - 50 MPa (abs), and gas hourly space velocity (GHSV) of 16 000 - 48000 h-¹ (Angew. Chem. Int. Ed. 2016, 55, 6261 -6265).

The reaction of hydrogen and carbon dioxide in step g) can be carried out in the presence of a copper-zinc-alumina catalyst. If copper-zinc-alumina catalysts are employed, the preferred temperature is in the range of from 150 to 300°C, preferably 175 to 300°C and the preferred pressure is in the range of from 10 to 150 bar (abs).

In step i) of the inventive processes, ethylene and/or propylene are manufactured by a methanol to olefin-process (MTO-process) from methanol obtained in step h).

A preferred process for the manufacture of ethylene and/or propylene from methanol and optionally ethanol comprises the steps:

A) feeding a methanol and optionally ethanol comprising feed stream A in a dimethyl ether fixed bed reactor and catalytic conversion of methanol to give dimethyl ether, wherein a product stream A1 comprising dimethyl ether, methanol, water vapor and optionally ethanol and ethylene is obtained;

B) mixing of the stream A1 with at last one hydrocarbon recycle stream R comprising C2-C6- hydrocarbons and catalytic conversion in an olefin fixed bed reactor to yield a raw product stream B comprising C2-C4-olefines, Cs-Ce-hydrocarbons und C7⁺-hydrocarbons;

C) cooling of the raw product stream B, wherein a hydrocarbon raw product stream C is obtained;

D) separating the hydrocarbon raw product stream C in a propylene containing value product stream, optionally an ethylene containing value product stream, a butene containing product stream, at least a Cs-Ce-hydrocarbon containing recycle stream and at least one Ce⁺-hydrocar- bons containing side product stream;

E) recycling of a part of the C2-C4-olefins and at least a part of the Cs-Ce-hydrocarbons as one or more hydrocarbon recycle streams R in step B);

F) recovering a propylene containing value product stream, an ethylene containing value product stream and optionally a butene containing value product stream;

G) discharging the Ce⁺-hydrocarbons containing side product stream.

The Ce⁺-hydrocarbons containing side product stream discharged in step G) can be further processed, preferably together with the lower boiling hydrocarbons obtained in step c) of the process of the invention, in a cracker, preferably a steam cracker, to give ethylene and/or propyl- ene. The ethylene and propylene obtained by the methanol to olefin-process is polymerized to give virgin polyethylene and polypropylene. The recycle loop is thereby closed.

Claims

1 . A process for the recycling of plastic waste containing at least one of polyethylene or polypropylene comprising the steps: a) thermal pyrolysis in an inert atmosphere of the plastic waste to obtain a pyrolysis oil, b) optionally purifying the pyrolysis oil obtained in step a), c) fractionating the pyrolysis oil to obtain at least one fraction of lower boiling hydrocarbons that can be further processed in a cracker, in particular a steam cracker, to give hydrocarbons of lower molecular weight, and at least one fraction of high-boiling residues, d) incinerating high-boiling residues obtained in step c) with an oxygen containing gas, wherein a carbon dioxide containing flue gas stream is obtained, e) purifying the carbon dioxide containing flue gas stream obtained in step d), wherein a purified carbon dioxide containing gas stream is obtained, f) reduction of the carbon dioxide contained in the gas stream obtained in step e) to obtain a gas stream containing carbon monoxide, optionally carbon dioxide and optionally hydrogen, g) optionally admixing hydrogen, preferably produced by water electrolysis, to the gas stream obtained in step f), h) reacting a gas mixture containing carbon monoxide, hydrogen and optionally carbon dioxide obtained in step f) or g) to give methanol, i) manufacturing ethylene and/or propylene by a methanol to olefin-process from methanol obtained in step h), j) polymerizing ethylene and/or propylene manufactured in step i) to give polyethylene and/or polypropylene, respectively.

2. The process according to claim 1 , wherein the pyrolysis oil obtained in step a) is purified in order to at least partly remove inorganic compounds and/or heteroatom containing organic compounds.

3. The process according to claim 1 or 2, wherein lower boiling hydrocarbons obtained in step c) are further processed in a cracker, in particular a steam cracker, to give ethylene and/or propylene.

4. The process according to claim 3, wherein ethylene and/or propylene are polymerized to give polyethylene and/or polypropylene.

5. The process according to any one of claims 1 to 4, wherein the high-boiling residue is incinerated in step d) with essentially pure oxygen as oxygen containing gas.

6. The process according to any one of claims 1 to 5, wherein the oxygen containing gas used in step d) is obtained by water electrolysis.

7. The process according to any one of claims 1 to 6, wherein the electrical power necessary for water electrolysis is generated at least in part from non-fossil, renewable resources.

8. The process according to any one of claims 1 to 7, wherein reduction of the carbon dioxide in step f) is carried out as electrochemical reduction to obtain a gas stream containing carbon monoxide, optionally carbon dioxide and optionally hydrogen, and a gas stream containing oxygen.

9. The process according to claim 8, wherein the electrical power necessary for the electrochemical reduction of CO2 to CO in step f) is generated at least in part from non-fossil, renewable resources.

10. The process according to claim 8 or 9, wherein the oxygen containing gas stream obtained in step f) is used in step d).

11 . The process according to any one of claims 1 to 10, wherein heat generated in step d) is used to produce electrical power.

12. A process for the recycling of plastic waste containing at least one of polyethylene or polypropylene comprising the steps a) thermal pyrolysis in an inert atmosphere of the plastic waste to obtain a pyrolysis oil, b) optionally purifying the pyrolysis oil obtained in step a), c) fractionating the pyrolysis oil to obtain at least one fraction of lower boiling hydrocarbons that can be further processed in a cracker, in particular a steam cracker, to give hydrocarbons of lower molecular weight, and at least one fraction of high-boiling residues, d) incinerating high-boiling residues obtained in step c) with an oxygen containing gas, wherein a carbon dioxide containing flue gas stream is obtained, e) purifying the carbon dioxide containing flue gas stream obtained in step d), wherein a purified carbon dioxide containing gas stream is obtained, f) admixing hydrogen, preferably produced by water electrolysis, to the gas stream obtained in step e), g) reacting a gas mixture containing hydrogen and carbon dioxide obtained in step f) to give ethylene and/or propylene, or h) reacting a gas mixture containing hydrogen and carbon dioxide obtained in step f) to give methanol, and i) manufacturing ethylene and/or propylene by a methanol to olefin-process from methanol obtained in step h), j) polymerizing ethylene and/or propylene manufactured in step g) or i) to give polyethylene and/or polypropylene, respectively.

13. The process according to claim 12, wherein the pyrolysis oil obtained in step a) is purified in order to at least partly remove inorganic compounds and/or heteroatom containing organic compounds.

14. The process according to claim 12 or 13, wherein lower boiling hydrocarbons obtained in step c) are further processed in a cracker, in particular a steam cracker, to give ethylene and/or propylene.

15. The process according to claim 14, wherein ethylene and/or propylene are polymerized to give polyethylene and/or polypropylene.

16. The process according to any one of claims 12 to 15, wherein the high-boiling residue is incinerated in step d) with essentially pure oxygen as oxygen containing gas.

17. The process according to any one of claims 12 to 16, wherein the oxygen containing gas used in step d) is obtained by water electrolysis.

18. The process according to any one of claims 12 to 17, wherein the electrical power necessary for water electrolysis is generated at least in part from non-fossil, renewable resources.

19. The process according to any one of claims 12 to 18, wherein heat generated in step d) is used to produce electrical power.